X-rays are electromagnetic waves that are highly transmitted through objects and are frequently used for nondestructive, noncontact observation of internal structures of objects. As a conventional X-ray irradiation apparatus applicable to such fields, an X-ray tube, described in Patent Document 1 indicated below, is known. An X-ray generating unit of the X-ray tube described in Patent Document 1 has a tubular casing that houses a target, and an exhaust pipe, put in communication with an internal space, is mounted to the casing (see FIG. 4, etc., of Patent Document 1). In manufacturing the X-ray tube, vacuum is drawn from the internal space of the casing via the exhaust pipe. After vacuum drawing, the exhaust pipe is closed and the internal space that houses the target is put in a vacuum state (state of being depressurized to a predetermined degree of vacuum).
Patent Document 1: U.S. Pat. No. 6,229,876